1. Field of the Invention
The present invention relates to a scanning optical system utilized in a laser beam printer and the like, and in particular, relates to the image forming optical system thereof (f.theta. lens optical system).
2. Description of the Related Art
A resin-made lens element, which can be molded by large amounts, has frequently been used as an f.theta. lens element in a laser beam printer and the like. However, the change in the shape or refractive index of a resin material, caused by a change in temperature, is generally large. Therefore a device which utilizes such an optical plastic material suffers deteriorated performance due to the temperature change, such as deviation of an image forming position, the change in a position of a scanning spot (image height) in the main scanning direction, which have been paid attention to as problems to be solved.
In the prior art, even if at the design stage the values are determined to satisfactorily correct the f.theta. characteristics and field curvature, once the temperature changes, the f.theta. characteristics deteriorate, and subsequently, field curvature becomes worse, whereby the position of the image plane, especially the position of the image plane in the sub-scanning direction, is moved by a large amount, and hence, desired performance is not obtained.
There are products to cope with the above performance deterioration due to the temperature change, the products of which incorporate glass lens elements into a part of an optical system or provide a temperature detection mechanism to perform dynamic correction. However, glass lens elements are more expensive than resin-made lens elements, and a device, such as a temperature detection mechanism, causes a rise in the overall cost.